Miocene-Pliocene history is recorded in south-central Colorado by sediments deposited in subsiding basins bounded by fault-block mountains and by faulted sedimentary and volcanic deposits lying on a channeled late Eocene erosion surface of regional extent. The San Luis Valley and upper Arkansas Valley are en echelon segments of the Rio Grande trough that are constricted and faulted south of Salida at the northern end of the Sangre de Cristo Range. Great movements on the bounding faults during Neogene time are indicated by clastic and volcanic trough fill, which may be 10,000 m thick near Alamosa and 1,500 m thick near Salida, and by adjoining mountains which stand as much as 1,500 m above the valley floors. The Sangre de Cristo Range owes most of its present elevation to Neogene faulting that cut the Miocene-Pliocene Dry Union Formation and some volcanic deposits in Oligocene-Miocene paleovalleys. Valley fill in the Wet Mountain Valley graben is as much as 1,500 m thick, and at least the upper 300 m of it consists of pinkish beds correlated with the Santa Fe(?) Formation. The crest of the Wet Mountains was uplifted about 400 m above the valley to the west of the mountains and nearly 1,200 m above the high plains which lie to the east. To the north, Neogene faulting elevated the Rampart Range at the front of the Rocky Mountains, dropped the Fourmile Creek graben nearly 400 m, formed complex fault-bounded basins at the southern margin of South Park, and segmented volcanic deposits in paleovalleys that once crossed the upper Arkansas Valley.